Polymers are basically heterogeneous materials, heterogeneity being exhibited through a variety of ways such as distribution of chain lengths, differences in chemical composition from chain to chain, and through the architecture of the chain as tacticity or as in branched and cross-linked structures. Branching may be characterized as long chain branching (LCB) or short chain branching (SCB).
Temperature rising elution fractionation (TREF) is a technique for analysis of crystalline and semi-crystalline polymers by separating the fractions according to crystallizability. TREF has utility to evaluate structure heterogeneity and enables to characterize the distribution of SCB. Elution during TREF is governed by the melting of semi-crystalline polymers in the presence of a solvent. Fractionation is expected to run similar to solid state melting and can be quantitatively correlated with the solid state melting.
Preparative TREF (PTREF) is performed to isolate polymer fractions for further investigations. This technique has certain disadvantages. During this process, a high quantity of solvent for eluting the sample and also a high quantity of non-solvent, usually methanol, for precipitation of eluting fractions, are required. Also this technique is very time-consuming as it takes several days to perform a fractionation.
An improvement upon the PTREF method was the development of analytical temperature rising elution fractionation (ATREF). ATREF overcomes the limitations of preparative TREF by considerably reducing the column and sample size and by continuously monitoring the amount of the eluted polymer fractions, as a function of column temperature using an on-line detector. The ATREF technique provides a crystallizability fractionation profile, which is the concentration of species eluting at a specified temperature. In ATREF the polymers to be analyzed are dissolved in a suitable hot solvent and allowed to crystallize in a column by slowly reducing the temperature. The crystallized polymer sample is eluted from the column by slowly increasing the temperature of the column. The concentration of the polymeric solution eluted from the column is detected with an on-line detector. A TREF chromatogram curve is then generated. ATREF can be applied for analyzing crystalline and semi-crystalline polymers, such as (poly)propylene (PP), (poly)ethylene (PE) and their copolymers. For semi-crystalline polymers, ATREF also provides an elution profile for the species that cannot crystallize. This amorphous portion of the polymer appears as a first peak when the elution process is started.
Existing methods for ATREF typically use an infrared (IR) detector as concentration detector for eluted fractions. In general, IR detectors offer good baseline stability for both high variations of temperature and pressure during the elution process. However, this type of detector has a poor sensitivity and consequently requires the use of large amounts of polymers in analyses. Furthermore, IR detectors are not available on commercial equipment and are complicated to install.
Differential refractive index (DRI) detectors are a valuable alternative for IR detectors as they are more sensitive than IR detector. U.S. Pat. No. 4,798,081 describes a method for performing ATREF wherein detection is done using a DRI detector. The document relates to a method and apparatus for coupling high temperature continuous viscometry (HTCV) with analytical temperature rising elution fractionation (ATREF) for analysis of crystalline and semi-crystalline polymers. The method and apparatus provide a crystallinity (or density or branching number) versus weight percent profile concurrently with viscosity average molecular weight for each incremental crystalline fraction. The concentration detector may be a differential refractometer, for measuring relative changes in the concentration of the solution. However, this document describes a method for performing ATREF with DRI detector which is complex and which requires intensive manual operations.
Existing methods for ATREF require that the solvent flow through the column is stopped during the crystallization of the polymer. This unavoidably leads to a poor baseline stability upon restarting of the flow during the elution of the polymer.
There remains a great need in the art for a method and apparatus for ATREF analysis, which overcomes the drawbacks of the known methods. The present invention relates to an improved apparatus and method suitable for performing ATREF analyses.